clarify extensions

[olsonpm]

I had to look at the code for koa-send to find whether extensions required the period before. So I added an example in the readme here to clarify

Checklist

• I have ensured my pull request is not behind the main or master branch of the original repository.
• I have rebased all commits where necessary so that reviewing this pull request can be done without having to merge it first.
• I have written a commit message that passes commitlint linting.
  I spent a few minutes trying to find a way to lint without installing something locally. If you want contributers to lint via commitlint then I suggest installing it per their current documentation
• I have ensured that my code changes pass linting tests.
• I have ensured that my code changes pass unit tests.
• I have described my pull request and the reasons for code changes along with context if necessary.